Intracellular calcium is the mediator of cardiac muscle contraction and relaxation. A rise in intracellular calcium initiates contraction and a decline enables relaxation. Elevation of intracellular calcium may cause arrhythmias, which can result from spontaneous release of calcium from the sarcoplasmic reticulum. Moreover, drugs used to treat arrhythmias may have direct and indirect effects on intracellular calcium, which may then be involved in both the desirable and the undesirable effects of drug therapy. In this Project we propose to study the complex interactions of antiarrhythmic agents with the sarcoplasmic reticulum and intracellular calcium. One intracellular calcium release channel, the ryanodine receptor, is becoming well understood at both a structural and functional level. Evidence suggests that there are other intracellular calcium release channels, the inositol trisphosphate (IP3) mediated channels, whose roles in cardiac function are largely unexplored. Using a variety of subcellular and single cell techniques, we will identify the presence of each of these intracellular release channels in both atrium and ventricle and characterize these channels by isolation. We will assess the interaction of antiarrhythmic agents with these channels, using competitive ligand binding in subcellular fractions to measure specific receptor interactions and relate these effects to calcium uptake and release. Intact isolated myocytes will be studied using fluorescent calcium indicators to examine drug effects on release of calcium from intracellular stores and influx from the extracellular pool. The goal of this project is to provide a basis for understanding the interaction of existing and new antiarrhythmic agents with the ion channels that affect intracellular calcium, with emphasis on the intracellular calcium release channels. By combining studies at the subcellular and molecular level with studies at the single cell and multicellular level, far greater insight may be gained than through studies at any one level. It is the hope that our studies will provide a framework for the use of antiarrhythmic agents with greater safety and efficacy.